1. Field of the Invention
The present invention relates to analogue to digital converters of the dual slope type.
2. Description of Prior Art
A conventional dual slope analogue to digital converter includes an integrator, a comparator, a clock and a reference voltage of opposite polarity to an input signal voltage. The signal voltage is integrated for a predetermined time controlled by the clock. The integrator is then disconnected from the signal voltage and connected to the reference voltage. When the output voltage from the integrator reaches zero the integrator is disconnected from the reference voltage and the circuit is switched into a resting phase. The duration of the reference voltage connection, which is proportional to the mean value of signal during the integration period, is measured in digital form by counting pulses from the clock. The digital output is the required value of the analogue signal input.
Analogue to digital converters of this type are simple and cheap and the use of the same clock for all timing purposes self-compensates for many possible sources of error. However, there are still limitations which preclude their use for high precision applications. Among the problem areas are dielectric storage in capacitors used in integrators, detection of the integrator zero point and the obtaining of a smooth cross-over as the input signal changes sign.
The use of converters in which the signal is fed continuously to the integrator has been described in U.S. Pat. No. 4361831 and in U.K. Pat. No. 1434414. U.S. Pat. No. 4,361,831 describes a converter in which the signal is fed continuously to the integrator and an auxiliary signal, or signals, part of the time. However, in this scheme a more complicated logic system is required and one or other of the auxiliary signals is always in use.
U.K. Pat. No. 1434414, also discloses a converter in which signal and reference may be connected simultaneously to an integrator. However this system requires two level detectors (or comparators) and the superposition of a period signal upon the integrator output.